1. FIELD OF THE INVENTION
This invention relates to pressure sensors having therein transducers for operation in low pressure (i.e., about 1 to 5 inches of water) environments and, more specifically, to such pressure sensors having capacitive pressure transducers.
2. BRIEF DESCRIPTION OF THE PRIOR ART
Pressure sensing devices using capacitive pressure transducers are well known and have received substantial acceptance. An example of such prior art pressure sensing devices is set forth in the patent of Charboneau et al. (U. S. Pat. No. 4,716,492).
While such prior art pressure sensing devices using capacitive pressure transducers provide good results and at reasonable cost, it is always desirable to improve the accuracy of such devices as well as to diminish the cost of fabrication.
In such prior art transducers using a single fixed substrate and a flexible diaphragm, there is no allowance for a true gauge pressure measurement. Furthermore, inaccuracies in result are provided due to the sealing of about 6 psia between the diaphragm and the substrate. While it is readily apparent that a true gauge pressure can be provided by opening the space between the diaphragm and the substrate to atmospheric pressure, this approach has also caused problems. As pressure greater than atmospheric pressure is applied to the flexible diaphragm, the diaphragm moves toward the fixed substrate. Since two silver plates are deposited, one on the substrate and one on the diaphragm, a pressure variable capacitor is created. The major disadvantage to this approach is therefore that atmospheric contaminants, such as chlorine and hydrogen sulfide, will damage the silver plates, thereby ultimately rendering the device non-functional.
A further problem encountered in pressure sensors of the above described type is that extraneous axial pressures applied to either or both of the plates of the capacitive transducer can cause inaccuracies in the actual pressure measurement. Such extraneous pressures often result from pressures applied to one or both of the capacitive transducer elements by the housing for the sensor itself.
A yet further problem encountered in pressure transducers of the above described type is that it is desirable to minimize the cost of fabrication. A cause of higher fabrication cost is the use of a separate printed board for the electronics required to provide an appropriate electrical indication of the sensed pressure.
A still further problem encountered in the pressure transducers of the above described type is that forces from the packaging or housing are applied to the movable diaphragm through the gasket, thereby providing an extraneous axial pressure to the movable diaphragm which results in inaccuracies in measurement.